In recent years, in the field of image display devices, an optical device driven by current has been developed. The luminance of emitting light of an optical device driven by current varies with the strength of current flowing therethrough. For example, Organic Light-Emitting Diode (OLED) devices are display devices emitting light in pixels. Unlike liquid crystal display devices, the OLED devices are self light-emitting devices, and in a display device using OLED, color grades are achieved by controlling the magnitude of current in the OLED.
Like a liquid crystal display, there is a passive matrix system and an active matrix system for a drive system in an OLED display device. The passive matrix system is structurally simple but has a few drawbacks, such as being difficult to produce a large and high-resolution display device, so the active matrix system is under prevailing development. In the active matrix system, current in a light-emitting device arranged for each pixel is controlled by a drive transistor.
At present, in the design of an Active Matrix Organic Light-Emitting Diodes (AMOLED) display panel, particularly in the design of a large-size substrate, non-uniformity of current in OLEDs results from non-uniformity and instability in preparation process of manufacturing thin film transistors (TFTs) on the panel. In order to compensate for a threshold voltage shift (Vth shift) due to the non-uniformity of TFTs manufactured in a substrate production process and to overcome the shortcoming of a decline in stability of the TFTs due to a bias voltage caused by long-time switching on status of the TFTs, the design of a compensation circuit is desirable. In the prior art, a drive circuit with pure PMOS TFTs is used in which a valid potential output is at a low potential, but an OLED device should be switched off in processes of node initialization, threshold detection and data input. However for the pure PMOS circuit in which only PMOS TFTs are used, a TFT is switched on in the case that its gate is at a low voltage and switched off in the case that the gate is at a high voltage; and the pure PMOS drive circuit outputs a valid level typically at a low level, so it is necessary to invert a signal output from the pure PMOS drive circuit so as to have the OLED switched off. In the prior art, signal inversion is achieved by using a light-emitting control (EMIT) drive circuit.
For inversion of a low potential to a high potential, an inverter is proposed in the prior art, which is structured as illustrated in FIG. 1A. The inverter comprises two P-type TFT, wherein the first TFT has its gate connected with an input terminal IN, has its source connected with a high voltage signal (VGH) and has its drain connected with an output terminal (OUT). The second TFT has its gate and drain connected with a low voltage signal (VGL) and has its source connected with the output terminal OUT. FIG. 1B is a timing control diagram of the circuit illustrated in FIG. 1A. With reference to FIG. 1A and FIG. 1B, the first TFT is switched off when IN is at a high potential, and since the second TFT is diode connected, (the second TFT has both its gate and its drain connected with the low voltage signal VGL as mentioned above), a low potential which is higher than VGL by Vth is output by the output terminal OUT and when IN is at a low potential, both the first TFT and the second TFT are switched on, so a high potential is output by the output terminal OUT. However in the foregoing circuit, the output terminal OUT is connected with both VGH and VGL, and if the TFTs are switched on or off completely, then the output terminal OUT should be connected either with VGH or with VGL, and thus the voltage output by the output terminal should be either VGH as a high voltage or VGL as a low voltage. But the forgoing circuit suffers from the problem of both the two TFTs being switched on at the same time, and the voltage output by the output terminal OUT is at a potential between the high voltage and the low voltage due to voltage division. As a result, the high or low potential at which the voltage is output lies between the high voltage and low voltage, thus making the high or low potential output insufficient, resulting in a continuous power supply and increasing power consumption. In addition, the output potential is not sufficient (for example, an input voltage of −5V˜10V may generate an output voltage of −4.43V˜5.07V), failing to effectively control TFTs in a pixel, thus making the compensation circuit fail to operate effectively.